苏北地区的小麦入春以后,当日平均气温升到3℃时开始返青,由营养生长向生殖生长转化。此时,当极端气温降到-5～-7℃时,小麦就会出现轻霜冻害,最低气温低于-7℃时出现重霜冻害。据灌云县气象站20年(1959～1978)气象资料,3月上中旬出现重霜冻害的有7年,平均三年一遇,出现轻霜冻害的有6年,平均3～4年一遇。3月下旬末到4月上旬初,三麦幼穗分化进入中 Northern Jiangsu wheat into the spring, the average daily temperature rose to 3 ℃ began to turn green, from vegetative growth to reproductive growth. At this point, when the extreme temperature drops to -5 ~ -7 ℃, wheat will appear light frost damage, the minimum temperature below -7 ℃ heavy frost damage. According to meteorological data of Guanyun weather station for 20 years (1959 ~ 1978), heavy frost damage occurred in the middle of March with an average of 7 years, with an average of 3 years. There was a light frost damage for 6 years, an average of 3 to 4 years Case. Late March to early April early, three wheat spike differentiation into